(1) Field of the Invention
The present invention relates to an optical semiconductor device, particularly to an optical semiconductor device including a combination of one or more optical semiconductor devices, or a combination of an optical semiconductor device and electrical circuit elements formed on a semiconductor substrate.
(2) Description of the Prior Art
A semiconductor laser element, which is an optical semiconductor device, usually has the structure shown in FIG. 1. In FIG. 1, 11 is an n-type InP substrate; 12 is an n-type InP buffer layer equivalently functioning as a clad layer; 13 is an active layer consisting of InGaAsP; 14 is a p-type InP clad layer; 15 is an insulating layer consisting of silicon dioxide (SiO.sub.2); 16 is a p-side electrode; and 17 is an n-side electrode.
In such a semiconductor laser, the current injection region is a striped region defined by the insulating layer 15. A positive voltage is applied to the p-side electrode 16 while a negative voltage is applied to the n-side electrode 17, thereby the carrier is injected into the active layer 13, thus causing light to be emitted through recombination. The emitted light has a larger photon energy than the band gap energy of the active layer 13 and is combined within the active layer 13 which is sandwiched between the clad layers 12 and 14 which have a lower refractive index than the active layer 13.
However, the light, penetrating into the clad layers 12 and 14 and scattering at the interface between the active layer 13 and layers 12 and 14 (hereinafter referred to as leakage light), propagates through the InP clad layers 12 and 14. Since the band gap of InP is wider than that of InGaAsP of the active layer 13, the leakage light propagates within the InP crystal without any attenuation and then returns to the active layer 13 after it is reflected at the interface of crystals due to the Fresnel reflection effect of the electrodes 16 and 17, arranged on the elements. The returning light not only makes unstable oscillation of the pertinent semiconductor laser element but also spacially disturbs the light emitting pattern or makes the pattern unstable.
FIG. 2 shows the emitting light for a field pattern in a direction perpendicular to the junction of the crystal. The zig-zag portion indicates a disturbance of the emitting pattern generated when the returning light enters the active layer 13. The disturbance of the emitting light for the field pattern causes the coherence to be poor and causes, for example, deteriorated resolution during picture processing.
In a device containing semiconductor light emitting elements, optical semiconductor devices such as photo-transistors, FET's or electrical circuit elements are formed on the same substrate. The leakage light or returning light from the semiconductor light emitting elements is captured as noise by the optical semiconductor elements and FET's or electrical circuit elements formed adjacent to each other on the same substrate, and makes the elements unstable.
For example, when configuring an optical integrated circuit by forming a semiconductor laser element and a Schottky barrier type field effect transistor element which drives the semiconductor laser element, on the same substrate, the leakage light from the active region of the semiconductor laser element reaches the channel region of the transistor element and causes generation of unwanted carriers in the channel region. The generation of unwanted carriers results in malfunction of the pertinent transistor element. This phenomenon becomes more distinct when the distance between the semiconductor laser element and transistor element becomes small, that is, when integration density becomes high.
Conventional optical integrated semiconductor devices provide a problem in that it is difficult to realize a high integration density because the characteristics of electronic devices deteriorate by the light emitted from an optical semiconductor device, such as a laser device, and moreover the phenomenon becomes more distinct when integration density is improved.